This invention relates generally to multispectral imaging systems, and more particularly, to a dual wavelength band imaging system for producing a composite visible/thermal-infrared image of an object or a scene.
Although the human eye is responsive only to radiation concentrated in the relatively narrow visible light region of the electromagnetic spectrum and is capable of recognizing an object from its reflected light even under poor lighting conditions, all objects reflect or emit nonvisible radiation which is useful for identifying or otherwise characterizing the objects. For example, passive infrared (IR)-thermal imaging (thermography) utilizes the passively emitted thermal radiation from an object as for non-destructive testing, medical diagnosis, surveillance and military target identification, and guidance and tracking. However, it is difficult to recognize even familiar objects from images produced from the object's passively emitted thermal radiation in the infrared spectral region in the absence of familiar visible cues. Under the pressures of simulated or actual combat, even experienced pilots have been known to fire at a "hot" feature revealed by their FLIR (Forward Looking Infrared) display system without being able to identify the target. Even the highest resolution thermal imaging systems can do little to improve this situation because of the fundamental differences in the behavior of matter in the visible and infrared spectral regions. For example, glass and water are transparent to the human eye, but are virtually opaque in the thermal infrared region. Paints and semiconductors are visually opaque, but are partially transparent to infrared radiation. A further complication is that all matter glows to some extent at infrared wavelengths, and an infrared image is, in effect, a radiant temperature map of an object or scene.
Multispectral imaging systems which produce a composite visible/infrared image are known. Such systems have the advantage of combining in a single image both visible and thermal information and are useful in enabling thermal features in a complex scene to be positively located and identified. Previously known systems, however, suffer from several problems, a principle one being the difficulty in obtaining exact spatial registration of the visible and infrared images. In systems which employ separate optical, detection, and image processing and display systems for the visible and thermal-infrared spectral bands, some degree of parallax is unavoidable due mainly to the separate optical trains. While overlaid multispectral images may be formed using digital image processing techniques, it is difficult to achieve exact spatial registration in current digital image processing systems. Also, digital processing of high resolution images is usually not performed in real time because of the extensive computations required. Even systems which have a common optical train have difficulty in achieving spatial registration of the visible and infrared images due to the difficulty in focusing the widely separated visible and infrared wavelength bands. Thus, it is desirable to avoid these problems in a composite dual wavelength visible/thermal imaging system.
In U.S. Pat. No. 4,679,068 issued July 7, 1987, and assigned to the same assignee as this patent application, there is described and claimed a composite visible/thermal infrared imaging system. This multispectral imaging apparatus includes a hybrid visible/infrared radiation detector which is a common focal plane onto which is focused both the visible and the infrared radiation from a scene to provide separate signals to the video imaging portions of the system. These signals are converted to a visible black and white image of the scene and a step-tone false color infrared image of the scene which images are combined in exact spatial registration to produce a composite image.
It is further desirable to provide an improved composite dual wavelength visible/thermal infrared imaging system over that described in the above discussed U.S. Pat. No. 4,679,068 wherein the visible and infrared radiation from a scene are focused at first and second focal planes, respectively, which are located the same distance from the optical system and thus, allows the user a wider choice of visible or infrared detection means.